1. Field of the Invention
The present invention relates to an assisted braking method and system in a motor vehicle and more particularly to adapting emergency braking detection to sequenced braking recognition, in other words the recognition of braking operations which occur close together in time.
2. Description of Related Art
In a known manner, a conventional hydraulic braking installation in a vehicle comprises at least one hydraulic braking circuit, conventionally two, each of these braking circuits being associated with at least one wheel brake, conventionally with two wheel brakes.
These braking circuits are supplied, in the case of normal assisted braking, by a master cylinder which can be actuated by means of the brake pedal via a braking force amplifier, generally a vacuum brake booster. The latter comprises a working chamber which is separated by a movable piston from a low-pressure chamber maintained permanently at a low pressure, i.e. a negative pressure with respect to atmospheric pressure, and on which the application of a pedal force makes it possible to place the working chamber under atmospheric pressure and correspondingly amplify the pedal force via the force acting on the movable piston.
Moreover, to prevent the wheels from locking, something which may cause the vehicle to lose control, antilock braking systems (ABS) have been developed and are now very widespread. They allow the braking pressure in the wheel brake to be controlled while monitoring the speed of rotation of each of the wheels. It is thus possible to obtain stable dynamic vehicle behavior during braking, even heavy braking. This type of system comprises a hydraulic braking block with electronic control comprising inlet valves and outlet valves which can be controlled electrically and which are associated with the various wheel brakes, a pressure sensor representing the pressure in the master cylinder, and delivery pumps, for example electrically driven delivery pumps, associated with a respective braking circuit. Depending on the dynamic behavior of the monitored wheel, the corresponding braking pressure can be relaxed to prevent locking by discharging some hydraulic fluid from the wheel brake in question to the delivery pump, then increased again by means of the high-pressure hydraulic fluid coming from the outlet of the delivery pump or from an accumulator associated with this pump.
The systems of the ABS type are often coupled to vehicle dynamics control systems, for example of the electronic stability program (ESP) type or antislip regulation (ASR) type. These systems are integrated with the hydraulic block of the ABS device. These vehicle dynamics control systems make it possible to provide a stable dynamic vehicle behavior by acting on the wheel brakes without the brake pedal having been pressed by the driver of the vehicle.
With such an ESP braking system, it is known to employ a hydraulic brake assist (HBA) function whereby the braking is performed with the assistance of the hydraulic block. More precisely, when this HBA function is employed, the hydraulic block autonomously raises the pressure of the hydraulic fluid in the hydraulic block so as to optimize the braking by reaching the limit of activation of the ABS device as quickly as possible.
In the case of sudden braking (or emergency braking), that is to say when the brake pedal is, for example, subjected very rapidly to a considerable pressure by the driver, the vehicle, equipped with an ESP hydraulic block with HBA function, is braked with the assistance provided by the hydraulic block following determination of an emergency braking situation.
The conventional criteria used to determine these emergency braking situations are, for example, the pressure in the master cylinder and/or the pressure gradient in the master cylinder, which allow good characterization of emergency braking operations.
The disadvantage of a hydraulic braking system with a vacuum brake booster lies in the fact that, in the case of rapidly repeated emergency braking operations, the pressure difference between the two chambers of the brake booster does not have time to become reestablished. The braking pressure is thus increased progressively less by the brake booster and it becomes more and more difficult to reach the emergency braking activation threshold.
Document U.S. Pat. No. 6,361,126 describes a solution to this problem. The emergency braking method according to that document takes into account the difference between the measurement of the pressure in one of the two chambers and the atmospheric pressure, and also the gradient of this pressure difference. Hence, the method according to document U.S. Pat. No. 6,361,126 makes it possible to adapt the pressure threshold to be reached in the master cylinder for triggering emergency braking to four criteria (the pressure in the master cylinder, the gradient with respect to time of the pressure in the master cylinder, the difference between the pressure measured in one of the chambers of the vacuum brake booster and the gradient of this pressure difference), making it possible in particular to characterize the repeated emergency braking situations and consequently adapt the activation of emergency braking. However, this method has the disadvantage of requiring the use of means for measuring the pressure in one of the brake booster chambers and of means for comparing this measured pressure with the atmospheric pressure, thus increasing the cost of the device.